1. Field of the Invention
This invention relates generally to a novel apparatus for analog to digital conversion utilizing two oscillators and a variable impedance associated with one of the oscillators, which is effective to change the phase of that oscillator in response to an analog condition.
2. Description of Prior Art
Conditions which can be measured, (e.g., temperature, voltage, pressure, weight, distance, velocity, capacitance, etc.) are often digitized so that the measure can be expressed as discrete steps or digits. The first step in digitizing often includes the use of a transducer, a device that will convert energy from one form to another. For example, a piezoelectric crystal can be used to convert pressure variations into an analog voltage and a thermister can be used to generate an analog voltage as a function of temperature. These voltages can then be digitized by an analog-digital converter. Other transducers, however, can more directly generate a digital output. For example, an interferometer can convert a displacement into a changing optical interference pattern that can then be converted into a pulsing, and hence digital, voltage by a photocell.
A popular form of analog-digital converter utilizes a ladder of matched resistors to divide either the input voltage or a reference voltage into a series of levels and arrive at a digitized or quantized representation of that voltage. When used in these devices, feedback is in the form of a voltage.
Voltage-to-frequency converters have a frequency output which is continuously variable. To be more useful, this output can be counted by a binary counter gated by a fixed clock or the output can be used to gate a fixed clock being used to drive a counter. Voltage-to-time converters such as integrators, can be used in a similar manner as a substitute for the voltage-to-frequency converters.
Another form of converter is the deltamodulator which generates a single weighed digital pulse train with the polarity of the pulses dependant upon a difference signal. This difference signal is created by subtracting the input voltage from the feedback voltage. The feedback voltage is generated by either a charge dispensing circuit or a digital-to-analog converter.
Yet another form of analog-digital converter represents a significant improvement over the analog-digital converters above described. This analog-digital converter is described in co-pending application filed by Robert G. Nelson on Dec. 28, 1983 bearing Ser. No. 566,314 and entitled "Methods and Apparatus for Analog-to-Digital Conversion". The analog-digital converter described therein employs two high-frequency oscillators having a digital output with the frequencies of the oscillators bearing a harmonic relationship one to the other. A representation of an analog condition is applied to cause a shift in phase in at least one of the digital signals. A comparator is then utilized to obtain a measure of the phase shift and the measure of phase shift is employed to adjust the phase of the phase shifted signal toward its original phase condition. The measure of phase shift results in a generation of a single weighed digital function representative of the amplitude of the adjustment utilized to return the phase shifted signal towards its original phase shifted condition. The single weighed digital function is applied to a low-pass digital filter to produce a multi-bit digital word representative of the instantaneous value of the analog condition.